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SubjectRe: [PATCH v2] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt
On Tue, 7 Dec 2021 23:23:03 +0000
Sean Christopherson <seanjc@google.com> wrote:
> On Tue, Nov 23, 2021 at 10:00 PM Wanpeng Li <kernellwp@gmail.com> wrote:
> > ---
> > arch/x86/kvm/lapic.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 759952dd1222..8257566d44c7 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -113,14 +113,13 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> >
> > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> > {
> > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > + return pi_inject_timer && kvm_mwait_in_guest(vcpu->kvm) && kvm_vcpu_apicv_active(vcpu);
>
> As Aili's changelog pointed out, MWAIT may not be advertised to the guest.
>
> So I think we want this? With a non-functional, opinionated refactoring of
> kvm_can_use_hv_timer() because I'm terrible at reading !(a || b).
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 40270d7bc597..c77cb386d03d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -113,14 +113,25 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
>
> static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> {
> - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> + (kvm_mwait_in_guest(vcpu) || kvm_hlt_in_guest(vcpu));
> }
>
> bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> {
> - return kvm_x86_ops.set_hv_timer
> - && !(kvm_mwait_in_guest(vcpu->kvm) ||
> - kvm_can_post_timer_interrupt(vcpu));
> + /*
> + * Don't use the hypervisor timer, a.k.a. VMX Preemption Timer, if the
> + * guest can execute MWAIT without exiting as the timer will stop
> + * counting if the core enters C3 or lower. HLT in the guest is ok as
> + * HLT is effectively C1 and the timer counts in C0, C1, and C2.
> + *
> + * Don't use the hypervisor timer if KVM can post a timer interrupt to
> + * the guest since posted the timer avoids taking an extra a VM-Exit
> + * when the timer expires.
> + */
> + return kvm_x86_ops.set_hv_timer &&
> + !kvm_mwait_in_guest(vcpu->kvm) &&
> + !kvm_can_post_timer_interrupt(vcpu));
> }
> EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);
>

It seems Sean and Wanpeng are busy with some other more important issues;
So Please let me try to merge Sean, Wanpeng's ideas and suggestions together,also including my opinions
into one possible approach and get it reviewed, Only if others are OK with this;

I will post a new patch for this later today or tomorrow.

Thanks!

--Aili Yao

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